Apparatus for measuring the mass of a flowing medium

ABSTRACT

An apparatus for measuring the mass of a flowing medium, in particular, for measuring the aspirated air mass in internal combustion engines. The apparatus includes a flow conduit for the medium, embodied as an air intake tube and having a restricted section, into which a bypass line toward the restricted section discharges at a mouth. In the bypass line there is a temperature-dependent resistor, which ascertains the mass of air flowing via the bypass line, this mass being at a predetermined proportion to that flowing through the flow conduit of the medium, and this resistor via an electronic control unit triggers a fuel-injection valve in the flow conduit of the medium. The course of the flow cross section in the restricted section is determined by way of example by a shaped body surrounding the fuel injection valve. The mouth of the bypass line is located at the narrowest flow cross section of the restricted section, while downstream of the narrowest cross section and adjacent thereto, the flow cross section is enlarged by an amount which corresponds to the bypass line cross section at the mouth. As a result, undesirable falsification of the measurement signal at the temperature-dependent resistor which might be caused by pulsations in the air flow is prevented.

BACKGROUND OF THE INVENTION

The invention relates to an apparatus for measuring the mass of aflowing medium, and, in particular, to an apparatus for measuring theaspirated air mass in an internal combustion engine. The apparatusincludes a flow conduit for the medium which has a restricted section,and a bypass line which has a mouth connected to the restricted sectionof the flow conduit, through which a mass of the medium flows which isat a predetermined proportion with respect to a mass of the mediumflowing through the flow conduit and discharges into the restrictedsection of the flow conduit. In this apparatus, the temperature and/orresistance of at least one temperature-dependent resistor disposedwithin the bypass line is regulated in accordance with the flowing massof medium, wherein the control variable of the temperature-dependentresistor is a standard for the flowing mass of medium.

Such an apparatus for measuring the mass of a flowing medium is alreadyknown; however, when this known apparatus is used for measuring the massof air aspirated by an internal combustion engine, the pulsations in theaspirated air, which are particularly pronounced in certain operatingranges, cause a falsification of the measurement signal. This isparticularly caused by the fact that in a restricted section, such asVenturi or a nozzle in the air intake tube of the engine, the pressuredrop is greater with a pulsating flow than with a flow which is free ofpulsation. Since the mass of air flowing through the bypass line isdependent on the pressure drop in the restricted section, measurementerrors result from the pressure drop dependent upon the pulsation, sincethe signal generated by the air meter is not only a function of theaspirated air mass but also a function of the amplitude of thepulsation.

SUMMARY OF THE INVENTION

In the apparatus for measuring the mass of a flowing medium, accordingto the invention, the restricted section of the flow conduit has a flowcross section such that, in the flow direction, it is enlarged to theleast extent at the mouth of the bypass line and thereafter by an amountwhich corresponds to the bypass line cross section at its mouth. Thisapparatus according to the invention has the advantage over the priorart that the falsifying effect on the result of measurement is at theleast reduced greatly.

The invention will be better understood and further objects andadvantages thereof will become more apparent from the ensuing detaileddescription of preferred embodiments taken in conjunction with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a circuit diagram of an apparatus for measuring the mass ofa flowing medium;

FIG. 2 is a schematic illustration of a fuel-injection system having afirst exemplary embodiment of an apparatus for measuring the mass of aflowing medium, for instance, the mass of air aspirated by an internalcombustion engine

FIG. 3 shows a second exemplary embodiment of an apparatus for measuringthe mass of a flowing medium; and

FIG. 4 shows a third exemplary embodiment of an apparatus for measuringthe mass of a flowing medium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 a flow cross-section 1, for instance a bypass line such asthat shown in FIGS. 2-4, is shown, in which a medium, for instance aportion of the air aspirated by an internal combustion engine, flows inthe direction of the arrows 2. A temperature-dependent resistor 3, forinstance a hot coating or film resistor or a hot wire, is located inthis flow cross section 1 and experiences the flow through it of theoutlet variable of a regulator, simultaneously furnishing the inputvariable for the regulator. The temperature of the temperature-dependentresistor 3 is adjusted by the regulator to a fixed value which is abovethe average air temperature. Now if the flow velocity, that is, the massof air aspirated per unit of time, increases, then thetemperature-dependent resistor 3 cools down to a greater extent. Thiscooling is fed back to the input of the regulator so that the outputvariable of the regulator is increased to such an extent that the fixedtemperature value is again established at the temperature-dependentresistor 3. The output variable of the regulator adjusts the temperatureof the temperature-dependent resistor 3 to the predetermined valuewhenever there are changes in the mass of aspirated air and at the sametime represents a standard for the mass of aspirated air which can befed as a measurement variable to a metering circuit at the internalcombustion engine for adapting the required mass of fuel to the mass ofair aspirated per unit of time.

Together with a resistor 4, the temperature-dependent resistor 3 forms afirst bridge branch, and a second bridge branch comprising the two fixedresistors 5 and 6 is connected in parallel to it. One pick-up point 7 islocated between the resistors 3 and 4, and another pick-up point 8 islocated between resistors 5 and 6. The two bridge branches are connectedin parallel at points 9 and 10. The diagonal voltage of the bridgeappearing between points 7 and 8 is supplied to the input of anamplifier 11, to the output terminals of which the points 9 and 10 areconnected, so that its output variable supplies the bridge withoperating voltage or operating current. The output variable, symbolizedas the control variable U_(s), can be picked up between the terminals 12and 13, as shown in FIG. 1. The control variable U_(s) controls themetering of the fuel required for the aspirated air, for instance, in afuel metering circuit, not shown, of an engine. Thetemperature-dependent resistor 3 is heated by the current flowingthrough it up to a value at which the input voltage of the amplifier 11,that is, the bridge diagonal voltage, becomes zero or assumes apredetermined value. A specific current then flows from the output ofthe amplifier into the bridge circuit. If a change in the mass ofaspirated air causes the temperature of the temperature-dependentresistor 3 to change, then the voltage at the bridge diagonal alsochanges and the amplifier 11 controls the bridge supply voltage orbridge current to a value at which the bridge is again balanced or isimbalanced in a predetermined manner. The output variable of theamplifier 11, the control voltage U_(s), like the current in thetemperature-dependent resistor 3, represents a standard for the mass ofaspirated air.

In order to compensate for the influence of temperature of the aspiratedair on the result of measurement, it may be efficaceous to include asecond resistor 14, which experiences the flow around it of aspiratedair, in the second bridge branch. The size of the resistors 5, 6 and 14should be selected such that the lost output of thetemperature-dependent resistor 14, caused by the branch current flowingthrough it, is low enough that the temperature of this resistor 14 willvirtually not vary with variations in the bridge voltage by ratheralways corresponds to the temperature of the aspirated air flowingaround it.

As shown in FIG. 2, the apparatus for measuring the mass of a flowingmedium can be used for controlling a fuel supply system for internalcombustion engines. In the exemplary embodiment shown in FIG. 2, thecombustion air flows in the direction of the arrow through an annularair filter 18 disposed in a housing 17 and into a flow conduit for themedium formed by an air intake tube 19. One or more cylinders, notshown, of a mixture-compressing internal combustion engine havingexternally supplied ignition communicates with this flow conduit for themedium. Near the air filter 18, the air intake tube 19 includes thereina coaxially disposed shaped body 25 which in combination therewith formsa restricted section 20 which in particular has a flow cross sectiontaking a Venturi-like course for the aspirated air. Downstream of therestricted section 20 there is a throttle device which is embodied as athrottle valve 21 and is arbitrarily actuatable. The bypass line 1branches off toward the restricted section 20 from the hollow chamber 22in the interior of the air filter 18 and the upstream side of its mouth23 is located at the narrowest flow cross section 34 of the restrictedsection 20. A mass of air which is set at a predetermined proportionwith respect to the mass of air flowing through the air intake tube 19flows through the bypass line 1. The temperature-dependent resistor 3,whose temperature and resistance values are regulated in accordance withthe aspirated air mass, serves the purpose of ascertaining the mass ofair aspirated by the engine. The flow cross section for the aspiratedair is determined in the restricted section 20 by the contour of thecoaxially disposed shaped body 25, by way of example, which surrounds afuel-injection valve 26. The fuel-injection valve 26 is triggered inaccordance with the aspirated air mass and other operating variables inthe engine, such as temperature, rpm, load, exhaust gas composition, andothers. The fuel supply of the fuel-injection valve 26 is effected byway of example by a fuel-pump 28 driven by an electric motor 27. Thefuel pump 28 aspirates fuel from a fuel container 29 and delivers it viaa fuel supply line 30 to the fuel injection valve 26. A line 31 in whicha pressure regulating valve 32 is disposed branches off from the fuelsupply line 30 and by way of this line 31 fuel can flow back to the fuelcontainer 29.

In accordance with the invention, the mouth 23 of the bypass line 1 islocated at the narrowest flow cross section 34 of the restricted section20; preferably, the upper edge 35 of the mouth 23 pointing upstream isat approximately the same level as the narrowest cross section 34. As aresult, the greatest pressure difference for the air flow through thebypass line 1 is available for attaining a maximally high measurementsignal at the temperature-dependent resistor 3. In order to preventpulsation from influencing the measurement signal, the flow crosssection for the aspirated air is widened in accordance with theinvention downstream of the narrowest cross section 34 by an amountwhich corresponds to the bypass line cross section at the mouth 23 thatis, the length of the spacing along line 34 plus a measure of thediameter of the mouth 23 is approximately equal to the spacing betweenthe wall of the intake tube 19 and the end of the shaped body 25downstream of the mouth 23. The branching off of the bypass line 1 fromthe hollow chamber 22 in the air filter 18, where there are fewpulsations, and the widening of the flow cross section at the mouth 23of the bypass line in accordance with the invention result in avirtually pulsation-free flow in the bypass line 1. The widening of theflow cross section in the restricted section 20 downstream of thenarrowest cross section 34 by the amount of the bypass line crosssection at the mouth 23 can be accomplished continuously beginning atthe level of the upper edge 35 of the mouth and down to the downstreamlower edge 36 of the mouth 23 or may be effected abruptly directlydownstream of the narrowest flow cross section 34.

In the exemplary embodiment of FIGS. 3 and 4, elements which remain thesame as and function the same as those in FIGS. 1 and 2 are identifiedby the same reference numerals. In the exemplary embodiment of FIG. 3, awidening of the flow cross section is again provided, as in theexemplary embodiment of FIG. 2, downstream of the narrowest crosssection 34 of the restricted section 20, the widening being effected bythe amount of the bypass line cross section at the mouth 23. Thissection is followed by a section 37 of the restricted section 20 inwhich, while the same flow cross section is maintained as at the loweredge 36 of the mouth 23, the walls of the restricted section 20 and ofthe shaped body 25 extend parallel to one another as far as the dot-dashline 38.

In the exemplary embodiment shown in FIG. 4, the restricted section 20is formed by the wall of the air intake tube 19. The narrowest flowcross section 34 is provided at the upper edge 35 of the mouth 23 of thebypass line 1, while downstream of the narrowest flow cross section 34an abrupt or continuous widening of the flow cross section by the amountof the bypass line cross section at the mouth 23 is effected.

If restriction of the flow cross section exists in the bypass line 1 andthe flow conduit 19 for the medium, then with different pulsationamplitude the proportions between the air masses flowing through thebypass line 1 and through the flow conduit 19 for the medium differ fromone another in an undesirable way. The result is that thetemperature-dependent resistor 3 will produce a false measurementresult. Such restriction could have a nozzle shape or a Venturi shape,by way of example. An error in measurement of this kind can be madesmaller by providing that such a restriction in the bypass line 1 and/orin the flow conduit 19 of the medium, will have only a convergingsection and not a diverging section. For instance, the bypass line 1 inFIG. 4 is shown with a flow cross section which continuously decreasesin the direction toward the mouth 23, while the transition downstream ofthe mouth 23 takes the form of a stepped widening.

The foregoing relates to preferred exemplary embodiments of theinvention, it being understood that other embodiments and variantsthereof are possible within the spirit and scope of the invention, thelatter being defined by the appended claims.

What is claimed and desired to be secured by Letters Patent of theUnited States is:
 1. In an apparatus for measuring the mass of a flowingmedium, which includes a flow conduit for the medium which has arestricted section, a bypass line which has a mouth connected to saidrestricted section and through which a mass of the medium flows which isat a predetermined proportion with respect to a mass of the mediumflowing through the flow conduit and discharges from said mouth intosaid restricted section, at least one temperature-dependent resistordisposed in the bypass line, and a regulating means for regulating thetemperature and/or resistance of the at least one temperature-dependentresistor, in accordance with the flowing mass of medium wherein acontrol variable of the at least one temperature-dependent resistor, isa standard for the flowing mass of medium, the improvement wherein saidrestricted section includes a shaped body which experiences a mediumflow around it, said shaped body being disposed coaxially within thesaid flow conduit to form said restricted section of the flow conduitfor the medium and is embodied such that the flow cross section of themedium in the flow direction becomes smaller up to the mouth of thebypass line and beyond the mouth of the bypass line widens by an amountwhich corresponds to the bypass line cross section at the mouth, andsaid shaped body is embodied such that the widening of the flow crosssection for the medium by the amount of the bypass line cross section atthe mouth of the bypass line is effected over a length extending from anupper edge to a lower edge of the mouth in the flow direction.
 2. Anapparatus as defined by claim 1 in which the shaped body surrounds afuel injection valve.
 3. An apparatus as defined by claim 2, whereindownstream of the mouth of the bypass line, a section of the flowconduit of the medium is provided in which, while the same flow crosssection for the medium is maintained, the wall of the flow conduit forthe medium and the wall of the shaped body extend parallel to oneanother.
 4. An apparatus as defined by claim 1 in which the restrictedsection is formed by a course of the wall of the medium flow conduit andsaid shaped body which is approximately Venturi-like in shape, the flowcross section for the medium of which is reduced in the flow directionup to the mouth of the bypass line and beyond the mouth of the bypassline is widened by an amount which corresponds to the bypass line crosssection at the mouth.